The invention is directed to improvements in a piezoelectric actuator, in particular for actuating control valves or injection valves in internal combustion engines in motor vehicles, having a piezoelectric actuator body, in particular in the form of a multilayer laminate of layered plies of piezoelectric material and intervening metal or electrically conductive layers acting as electrodes, these electrode layers being contacted by electrically conductive common electrode leads, and one of the face ends of the actuator body is fixed on an actuator base, and the actuator body is surrounded by a module wall while maintaining an interstice between them.
One such piezoelectric actuator is disclosed in German Patent Disclosure DE 196 50 900 A1 of Robert Bosch GmbH.
As is well known, piezoelectric actuators can for instance be used for injection valves of a vehicle motor and in brake systems with anti-lock and traction control systems.
Such injection valves equipped with piezoelectric actuators have an injection nozzle controlled by a tappetlike closure device. An operative face toward the nozzle is disposed on the tappet and is acted upon by the pressure of the fuel supplied to the nozzle; the pressure forces seek to urge the tappet in the opening direction of the closure device. The tappet protrudes with a plungerlike end, whose cross section is larger than the aforementioned operative face, into a control chamber. The pressure effective there seeks to urge the tappet in the closing direction of the closure device. The control chamber communicates with the fuel supply, which is at a high pressure, via an inlet throttle and with a fuel return line that has only low pressure, via an outlet valve that is throttled as a rule or is combined with an outlet throttle. When the outlet valve is closed, a high pressure prevails in the control chamber, by which the tappet is moved in the closing direction of the closure device, counter to the pressure on its operative face toward the nozzle, or is kept in the closing position. Upon opening of the outlet valve, the pressure in the control chamber drops; the magnitude of the drop in pressure is determined by the size of the inlet throttle and by the throttle resistance of the opened outlet valve, or the outlet throttle combined with it. As a result, the pressure in the control chamber decreases when the outlet valve is opened, in such a way that the tappet is moved in the opening direction of the closure device, or held in the open position, by the pressure forces that are operative on its operative face toward the nozzle.
In comparison with electromagnetically actuated injection valves, piezoelectric actuators can switch faster. However, in the design of a piezoelectric actuator, it must be noted that internal losses in the piezoelectric body of the actuator cause lost heat, which has to be dissipated so that the actuator will not overheat. Since the ceramic materials of the piezoelectric ceramic have poor heat conductivity, the dissipation inside the actuator body, which substantially comprises ceramic material, is unfavorable.
The heat generated by the actuator backs up in the interstice between the actuator body and the module wall, if only air is present in the interstice.
Cooling the actuator with a liquid coolant, such as fuel, water, motor oil and the like, which is theoretically possible, is unfavorable, first because of the risk of a short circuit from the water component that is contained both in the fuel and in motor oil, and second because the actuator module is more expensive because of complicated seals, to preclude leakage of the coolant used, especially when the actuator becomes heated.
It is therefore the object of the invention to make a generic piezoelectric actuator possible in such a way that cooling during operation is possible without a liquid coolant such as motor oil, water or fuel; that the piezoelectric actuator can be installed simply, and that no special seals as in liquid cooling are necessary.
A piezoelectric actuator according to the invention thus has the advantage that the heated actuator body can be cooled without fuel, motor oil or water, that is, without the risk of causing a short circuit because of the water component, and also without the risk of leakage.
According to the invention, to that end the interstice between the actuator body and the module wall is filled with an elastic or plastic electrically insulating material with good heat conductivity, which at least in the temperature range below the operating temperature of the actuator is solid. With this kind of material as a cooling medium, the risk of leakage disappears.
Cooling from thermal conduction through the interstice, filled with the electrically insulating solid, radially directly into the module wall, which for instance is of metal, enhances the cooling effect in comparison with thermal conduction exclusively to the actuator base.
Advantageously, gas or air bubbles are admixed with the material; they prevent a major pressure development in the interior of the actuator module.
The viscosity of the material used as the cooling medium can be selected either in such a way that it does change to a fluid liquid state even when heated, or alternatively in such a way that the material, such as a wax, changes to the liquid state at higher temperatures, which can be in the range between 50xc2x0 C. and 100xc2x0 C. The formation of the liquid phase of the material at elevated temperature intensifies the cooling, and the cooling remains sufficiently strong.
At room temperature and below, the actuator exhibits a somewhat shorter stroke. The phase of the shorter stroke is quickly completed as a result of faster heating.
In the solid state of the material, the cooling is only moderate; that is, by intrinsic heating, the actuator changes very quickly over to the operating state having the greater stroke, before the cooling then ensues by liquefication of the material.
Care is preferably taken to assure that the interstice of the piezoelectric actuator of the invention is not completely filled with material, so that when the actuator heats up, this material can expand into a chamber that contains air or gas.
So that no hindrance of the actuator stroke will ensue, the module wall, which for instance is of metal, is coated on the inside with an anti-stick coating, which prevents the material from adhering to the wall.
The surface of the actuator body can also be provided with an anti-stick coating of this kind, for instance of PTFE.
Also preferably, the actuator base is sealed off from the interstice that contains the material by a sealing agent, such as an adhesive, so that the liquefied material cannot escape from the actuator module.
Further characteristics and advantages of the invention will become clear below from the description of the ensuing exemplary embodiments of the invention taken in conjunction with the drawing.